In rotating a rolling bearing incorporating a resin-made cage at a high speed, a centrifugal force generated owing to a high-speed rotation acts on a cage. As a result, the cage deforms. Owing to the deformation of the cage, the friction between the cage and balls held by the cage becomes high, and the torque of the rolling bearing becomes high. An increase of the friction therebetween causes the bearing to generate heat. Further owing to the deformation of the cage, the cage may contact an outer ring of the bearing. Resin melts owing to frictional heat caused by the contact between the cage and the outer ring. Thereby there may be a case where the rolling bearing is prevented from rotating. Thus the resin-made cage to be incorporated in the rolling bearing which is used at a high-speed rotation is an important bearing member.
To restrain the cage from deforming when the rolling bearing rotates at a high speed, it is necessary to increase the mechanical strength and elastic modulus of a resin composition for the cage. To this end, normally this problem is dealt with by increasing the mixing amount of a fibrous reinforcing material such as glass fiber in the resin composition.
A “crown-type cage” is used as the resin-made cage of a deep groove ball bearing which is a kind of the rolling bearing. At a production time of the crown-type cage by injection molding, it is necessary to perform “forcible drawing” to take out a molded article from a semicircular molding die for forming a ball-holding space by spreading claws of the molded article immediately after injection molding finishes. The problem with the forcible drawing is that it is impossible for a resin composition containing a large mixing amount of the fibrous reinforcing material and thus having a low bending strength and a low bending strain (elongation property), particularly the resin composition consisting of polytrimethylene terephthalate (hereinafter referred to as PTT) to permit its own deformation which occurs at the time of the forcible drawing. As a result, troubles such as crack and whitening may occur on the inner-diameter surface of the pocket of the crown-type cage. Similar problems may occur at the time of forming a resin-made seal by injection-molding a resin material. Thus in forcibly drawing a lip from a molding die at a molding time, the resin material for the resin-made seal is incapable of permitting its own deformation and thus cannot be molded owing to the occurrence of the whitening and the crack.
As a measure for dealing with the forcible drawing, an example in which the configuration of the cage is devised to decrease a load at the time of the forcible drawing is reported (see patent document 1). But the cage having the proposed configuration cannot necessarily be so designed that the cage has a high strength. Another problem is that decreasing the load at the time of the forcible drawing in molding the resin material into the cage is synonymous with decreasing a steel ball-holding force of the cage incorporated in the bearing and is thus unpreferable. Therefore there is a demand for the development of a resin material having a high elastic modulus and a high elongation property to such an extent that the resin composition allows the forcible drawing to be accomplished at the molding time. Until now such a resin material does not exist.
The biodegradable plastic composition whose biodegradation speed is adjusted by adding the polycarbodiimide compound to the biodegradable plastic material is known (see patent document 2).
To improve the resistance to hydrolysis in a high-temperature acid medium, the polyamide resin composition containing the polyamide resin and the aliphatic or alicyclic carbodiimide added by 0.01 to 20 parts by mass to 100 parts by mass of the polyamide resin is known (see patent document 3).
To improve the resistance to the hydrolysis, oil, and halogenated metal salts in the high-temperature acid medium, the polyamide resin composition containing the polyamide resin and the aliphatic carbodiimide compound added thereto is known (see patent document 4).
To provide a resin composition excellent in the resistance to the hydrolysis, oil, and heat aging property, the thermoplastic copolymerized polyester resin composition containing the thermoplastic copolymerized polyester resin and the monocarbodiimide compound and/or the polycarbodiimide compound added by 0.01 to 10 parts by weight to 100 parts by weight of the thermoplastic copolymerized polyester resin is known (see patent document 5).
Conventionally the carbodiimide compound is contained in the resin composition to improve its chemical properties such as hydrolyzability. It is not known so far that the addition of the carbodiimide compound to a resin allows the production of a resin composition which has a high elastic modulus and a high elongation property to such an extent that resin composition receives a load applied thereto to a low extent at the time of the forcible drawing in molding it into the cage or the like and thus can be used as a bearing member.